CURious Variables Experiment (CURVE): CCD photometry of active dwarf nova DI Ursae Majoris^*

¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland e-mail: [rudy;olech;mwisniew;pietruk]@camk.edu.pl
² Departamento de Astronomia y Astrofisica, Pontificia Universidad Catolica de Chile, Casilla 306, Santiago 22, Chile
³ J. Pala's Amateur Observatory, Słupsk, Poland
⁴ Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 30 October 2008
Accepted: 7 January 2009

Abstract

Context. We report an analysis of photometric behaviour of DI UMa, an extremely active dwarf nova. The observational campaign (completed in 2007) covers five superoutbursts and four normal outbursts.

Aims. We examined principal parameters of the system to understand peculiarities of DI UMa, and other active cataclysmic variables.

Methods. Based on precise photometric measurements, temporal light curve behaviour,  analysis, and power spectrum analysis, we investigated physical parameters of the system.

Results. We found that the period of the supercycle now equals  days. Observations during superoutbursts infer that the period of superhumps equals  days ( min). During quiescence, the light curve reveals a modulation of period  days ( min), which we interpret as the orbital period of the binary system. The values obtained allowed us to determine a fractional period excess of , which is surprisingly small compared to the usual value for dwarf novae (2%–5%). A detailed analysis was performed for two superoutbursts with the most comprehensive coverage. In both cases, we detected an increase in the superhump period with a mean rate of .

Conclusions. Based on these measurements, we confirm that DI UMa is probably a period bouncer, an old system that reached its period minimum a long time ago, has a secondary that became a degenerate brown dwarf, the entire system evolving now toward longer periods. DI UMa is an extremely interesting object because we know only one more active ER UMa star with similar characteristics (IX Dra).